Methods of manufacturing engineered substrate structures for power and rf applications

ABSTRACT

A method of manufacturing a substrate includes forming a support structure by providing a polycrystalline ceramic core, encapsulating the polycrystalline ceramic core in a first adhesion shell, encapsulating the first adhesion shell in a conductive shell, encapsulating the conductive shell in a second adhesion shell, and encapsulating the second adhesion shell in a barrier shell. The method also includes joining a bonding layer to the support structure, joining a substantially single crystalline silicon layer to the bonding layer, forming an epitaxial silicon layer by epitaxial growth on the substantially single crystalline silicon layer, and forming one or more epitaxial layers by epitaxial growth on the epitaxial silicon layer.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 16/287,782, filed Feb. 27, 2019, which is a divisionalapplication of U.S. patent application Ser. No. 15/621,335 filed on Jun.13, 2017, now U.S. patent application Ser. No. 10,297,445, which claimsbenefit of U.S. Provisional Patent Application No. 62/350,084, filed onJun. 14, 2016, entitled “ENGINEERED SUBSTRATE STRUCTURE FOR POWER AND RFAPPLICATIONS,” the disclosures of which are hereby incorporated byreference in their entirety for all purposes.

BACKGROUND OF THE INVENTION

Light-emitting diode (LED) structures are typically epitaxially grown onsapphire substrates. Many products currently use LED devices, includinglighting, computer monitors, and other display devices.

The growth of gallium nitride based LED structures on a sapphiresubstrate is a heteroepitaxial growth process since the substrate andthe epitaxial layers are composed of different materials. Due to theheteroepitaxial growth process, the epitaxially grown material canexhibit a variety of adverse effects, including reduced uniformity andreductions in metrics associated with the electronic/optical propertiesof the epitaxial layers. Accordingly, there is a need in the art forimproved methods and systems related to epitaxial growth processes andsubstrate structures.

SUMMARY OF THE INVENTION

The present invention relates generally to engineered substratestructures. More specifically, the present invention relates to methodsand systems suitable for use in epitaxial growth processes. Merely byway of example, the invention has been applied to a method and systemfor providing a substrate structure suitable for epitaxial growth thatis characterized by a coefficient of thermal expansion (CTE) that issubstantially matched to epitaxial layers grown thereon. The methods andtechniques can be applied to a variety of semiconductor processingoperations.

According to an embodiment of the present invention, a substrate isprovided. The substrate includes a support structure comprising: apolycrystalline ceramic core; a first adhesion layer coupled to thepolycrystalline ceramic core; a conductive layer coupled to the firstadhesion layer; a second adhesion layer coupled to the conductive layer;and a barrier layer coupled to the second adhesion layer. The substratealso includes a silicon oxide layer coupled to the support structure, asubstantially single crystalline silicon layer coupled to the siliconoxide layer, and an epitaxial III-V layer coupled to the substantiallysingle crystalline silicon layer.

According to another embodiment of the present invention, a method ofmanufacturing a substrate is provided. The method includes forming asupport structure by: providing a polycrystalline ceramic core;encapsulating the polycrystalline ceramic core in a first adhesionshell; encapsulating the first adhesion shell in a conductive shell;encapsulating the conductive shell in a second adhesion shell; andencapsulating the second adhesion shell in a barrier shell. The methodalso includes joining a bonding layer to the support structure, joininga substantially single crystalline silicon layer to the bonding layer,forming an epitaxial silicon layer by epitaxial growth on thesubstantially single crystalline silicon layer, and forming an epitaxialIII-V layer by epitaxial growth on the epitaxial silicon layer.

According to a specific embodiment of the present invention, anengineered substrate structure is provided. The engineered substratestructure includes a support structure, a bonding layer coupled to thesupport structure, a substantially single crystalline silicon layercoupled to the bonding layer, and an epitaxial single crystal siliconlayer coupled to the substantially single crystalline silicon layer. Thesupport structure includes a polycrystalline ceramic core, a firstadhesion layer coupled to the polycrystalline ceramic core, a conductivelayer coupled to the first adhesion layer, a second adhesion layercoupled to the conductive layer, and a barrier shell coupled to thesecond adhesion layer.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, embodiments of the presentinvention provide an engineered substrate structure that is CTE matchedto gallium nitride based epitaxial layers suitable for use in optical,electronic, and optoelectronic applications. Encapsulating layersutilized as components of the engineered substrate structure blockdiffusion of impurities present in central portions of the substratefrom reaching the semiconductor processing environment in which theengineered substrate is utilized. The key properties associated with thesubstrate material, including the coefficient of thermal expansion,lattice mismatch, thermal stability, and shape control are engineeredindependently for an improved (e.g., an optimized) match with galliumnitride-based epitaxial and device layers, as well as with differentdevice architectures and performance targets. Because substratematerials layers are integrated together in the conventionalsemiconductor fabrication processes, process integration is simplified.These and other embodiments of the invention along with many of itsadvantages and features are described in more detail in conjunction withthe text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram illustrating an engineeredsubstrate structure according to an embodiment of the present invention.

FIG. 2A is a SIMS profile illustrating species concentration as afunction of depth for an engineered structure according to an embodimentof the present invention.

FIG. 2B is a SIMS profile illustrating species concentration as afunction of depth for an engineered structure after anneal according toan embodiment of the present invention.

FIG. 2C is a SIMS profile illustrating species concentration as afunction of depth for an engineered structure with a silicon nitridelayer after anneal according to an embodiment of the present invention.

FIG. 3 is a simplified schematic diagram illustrating an engineeredsubstrate structure according to another embodiment of the presentinvention.

FIG. 4 is a simplified schematic diagram illustrating an engineeredsubstrate structure according to yet another embodiment of the presentinvention.

FIG. 5 is a simplified flowchart illustrating a method of fabricating anengineered substrate according to an embodiment of the presentinvention.

FIG. 6 is a simplified schematic diagram illustrating anepitaxial/engineered substrate structure for RF and power applicationsaccording to an embodiment of the present invention.

FIG. 7 is a simplified schematic diagram illustrating a III-V epitaxiallayer on an engineered substrate structure according to an embodiment ofthe present invention.

FIG. 8 is a simplified flowchart illustrating a method of fabricating anengineered substrate according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Embodiments of the present invention relate to engineered substratestructures. More specifically, the present invention relates to methodsand systems suitable for use in epitaxial growth processes. Merely byway of example, the invention has been applied to a method and systemfor providing a substrate structure suitable for epitaxial growth thatis characterized by a coefficient of thermal expansion (CTE) that issubstantially matched to epitaxial layers grown thereon. The methods andtechniques can be applied to a variety of semiconductor processingoperations.

FIG. 1 is a simplified schematic diagram illustrating an engineeredsubstrate structure according to an embodiment of the present invention.The engineered substrate 100 illustrated in FIG. 1 is suitable for avariety of electronic and optical applications. The engineered substrateincludes a core 110 that can have a coefficient of thermal expansion(CTE) that is substantially matched to the CTE of the epitaxial materialthat will be grown on the engineered substrate 100. Epitaxial material130 is illustrated as optional because it is not required as an elementof the engineered substrate, but will typically be grown on theengineered substrate.

For applications including the growth of gallium nitride (GaN)-basedmaterials (epitaxial layers including GaN-based layers), the core 110can be a polycrystalline ceramic material, for example, polycrystallinealuminum nitride (AlN), which can include a binding material such asyttrium oxide. Other materials can be utilized in the core 110,including polycrystalline gallium nitride (GaN), polycrystallinealuminum gallium nitride (AlGaN), polycrystalline silicon carbide (SiC),polycrystalline zinc oxide (ZnO), polycrystalline gallium trioxide(Ga₂O₃), and the like.

The thickness of the core can be on the order of 100 to 1,500 μm, forexample, 725 μm. The core 110 is encapsulated in a first adhesion layer112 that can be referred to as a shell or an encapsulating shell. In anembodiment, the first adhesion layer 112 comprises a tetraethylorthosilicate (TEOS) layer on the order of 1,000 Å in thickness. Inother embodiments, the thickness of the first adhesion layer varies, forexample, from 100 Å to 2,000 Å. Although TEOS is utilized for adhesionlayers in some embodiments, other materials that provide for adhesionbetween later deposited layers and underlying layers or materials (e.g.,ceramics, in particular, polycrystalline ceramics) can be utilizedaccording to an embodiment of the present invention. For example, SiO₂or other silicon oxides (Si_(x)O_(y)) adhere well to ceramic materialsand provide a suitable surface for subsequent deposition, for example,of conductive materials. The first adhesion layer 112 completelysurrounds the core 110 in some embodiments to form a fully encapsulatedcore and can be formed using an LPCVD process. The first adhesion layer112 provides a surface on which subsequent layers adhere to formelements of the engineered substrate structure.

In addition to the use of LPCVD processes, furnace-based processes, andthe like to form the encapsulating first adhesion layer, othersemiconductor processes can be utilized according to embodiments of thepresent invention, including CVD processes or similar depositionprocesses. As an example, a deposition process that coats a portion ofthe core can be utilized, the core can be flipped over, and thedeposition process could be repeated to coat additional portions of thecore. Thus, although LPCVD techniques are utilized in some embodimentsto provide a fully encapsulated structure, other film formationtechniques can be utilized depending on the particular application.

A conductive layer 114 is formed surrounding the adhesion layer 112. Inan embodiment, the conductive layer 114 is a shell of polysilicon (i.e.,polycrystalline silicon) that is formed surrounding the first adhesionlayer 112 since polysilicon can exhibit poor adhesion to ceramicmaterials. In embodiments in which the conductive layer is polysilicon,the thickness of the polysilicon layer can be on the order of 500-5,000Å, for example, 2,500 Å. In some embodiments, the polysilicon layer canbe formed as a shell to completely surround the first adhesion layer 112(e.g., a TEOS layer), thereby forming a fully encapsulated firstadhesion layer, and can be formed using an LPCVD process. In otherembodiments, as discussed below, the conductive material can be formedon a portion of the adhesion layer, for example, a lower half of thesubstrate structure. In some embodiments, conductive material can beformed as a fully encapsulating layer and subsequently removed on oneside of the substrate structure.

In an embodiment, the conductive layer 114 can be a polysilicon layerdoped to provide a highly conductive material, for example, doped withboron to provide a p-type polysilicon layer. In some embodiments, thedoping with boron is at a level of 1×10¹⁹ cm⁻³ to 1×10²⁰ cm⁻³ to providefor high conductivity. Other dopants at different dopant concentrations(e.g., phosphorus, arsenic, bismuth, or the like at dopantconcentrations ranging from 1×10¹⁶ cm⁻³ to 5×10¹⁸ cm⁻³) can be utilizedto provide either n-type or p-type semiconductor materials suitable foruse in the conductive layer. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

The presence of the conductive layer 114 is useful during electrostaticchucking of the engineered substrate to semiconductor processing tools,for example tools with electrostatic chucks (ESC). The conductive layer114 enables rapid dechucking after processing in the semiconductorprocessing tools. Thus, embodiments of the present invention providesubstrate structures that can be processed in manners utilized withconventional silicon wafers. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

A second adhesion layer 116 (e.g., a TEOS layer on the order of 1,000 Åin thickness) 116 is formed surrounding the conductive layer 114. Thesecond adhesion layer 116 completely surrounds the conductive layer 114in some embodiments to form a fully encapsulated structure and can beformed using an LPCVD process, a CVD process, or any other suitabledeposition process, including the deposition of a spin-on dielectric.

A barrier layer 118, for example, a silicon nitride layer, is formedsurrounding the second adhesion layer 116. In an embodiment, the barrierlayer 118 is a silicon nitride layer 118 that is on the order of 2,000 Åto 5,000 Å in thickness. The barrier layer 118 completely surrounds thesecond adhesion layer 116 in some embodiments to form a fullyencapsulated structure and can be formed using an LPCVD process. Inaddition to silicon nitride layers, amorphous materials including SiCN,SiON, AlN, SiC, and the like can be utilized as barrier layers. In someimplementations, the barrier layer 118 comprises a number of sub-layersthat are built up to form the barrier layer. Thus, the term barrierlayer is not intended to denote a single layer or a single material, butto encompass one or more materials layered in a composite manner. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

In some embodiments, the barrier layer 118, e.g., a silicon nitridelayer, prevents diffusion and/or outgassing of elements present in thecore 110, for example, yttrium oxide (i.e., yttria), oxygen, metallicimpurities, other trace elements, and the like into the environment ofthe semiconductor processing chambers in which the engineered substratecould be present, for example, during a high temperature (e.g., 1,000°C.) epitaxial growth process. Utilizing the encapsulating layersdescribed herein, ceramic materials, including polycrystalline AlN thatare designed for non-clean room environments, can be utilized insemiconductor process flows and clean room environments.

FIG. 2A is a secondary ion mass spectroscopy (SIMS) profile illustratingspecies concentration as a function of depth for an engineered structureaccording to an embodiment of the present invention. The engineeredstructure did not include barrier layer 118. Referring to FIG. 2A,several species present in the ceramic core (e.g., yttrium, calcium, andaluminum) drop to negligible concentrations in the engineered layers120/122. The concentrations of calcium, yttrium, and aluminum drop bythree, four, and six orders of magnitude, respectively.

FIG. 2B is a SIMS profile illustrating species concentration as afunction of depth for an engineered structure without a barrier layerafter anneal according to an embodiment of the present invention. Asdiscussed above, during semiconductor processing operations, theengineered substrate structures provided by embodiments of the presentinvention can be exposed to high temperatures (˜1,100° C.) for severalhours, for example, during epitaxial growth of GaN-based layers.

For the profile illustrated in FIG. 2B, the engineered substratestructure was annealed at 1,100° C. for a period of four hours. As shownby FIG. 2B, calcium, yttrium, and aluminum, originally present in lowconcentrations in the as deposited sample, have diffused into theengineered layers, reaching concentrations similar to other elements.

FIG. 2C is a SIMS profile illustrating species concentration as afunction of depth for an engineered structure with a barrier layer afteranneal according to an embodiment of the present invention. Theintegration of the diffusion barrier layer 118 (e.g., a silicon nitridelayer) into the engineered substrate structure prevents the diffusion ofcalcium, yttrium, and aluminum into the engineered layers during theannealing process that occurred when the diffusion barrier layer was notpresent. As illustrated in FIG. 2C, calcium, yttrium, and aluminumpresent in the ceramic core remain at low concentrations in theengineered layers post-anneal. Thus, the use of the barrier layer 118(e.g., a silicon nitride layer) prevents these elements from diffusingthrough the diffusion barrier and thereby prevents their release intothe environment surrounding the engineered substrate. Similarly, anyother impurities contained within the bulk ceramic material would becontained by the barrier layer.

Typically, ceramic materials utilized to form the core 110 are fired attemperatures in the range of 1,800° C. It would be expected that thisprocess would drive out a significant amount of impurities present inthe ceramic materials. These impurities can include yttrium, whichresults from the use of yttria as sintering agent, calcium, and otherelements and compounds. Subsequently, during epitaxial growth processes,which are conducted at much lower temperatures in the range of 800° C.to 1,100° C., it would be expected that the subsequent diffusion ofthese impurities would be insignificant. However, contrary toconventional expectations, the inventors have determined that evenduring epitaxial growth processes at temperatures much less than thefiring temperature of the ceramic materials, significant diffusion ofelements through the layers of the engineered substrate can occur. Thus,embodiments of the present invention integrate a barrier layer 118(e.g., a silicon nitride layer) to prevent out-diffusion of thebackground elements from the polycrystalline ceramic material (e.g.,AlN) into the engineered layers 120/122 and epitaxial layers such asoptional GaN layer 130. The silicon nitride layer 118 encapsulating theunderlying layers and material provides the desired barrier layerfunctionality.

As illustrated in FIG. 2B, elements originally present in the core 110,including yttrium diffuse into and through the first TEOS layer 112, thepolysilicon layer 114, and the second TEOS layer 116. However, thepresence of the silicon nitride layer 118 prevents these elements fromdiffusing through the silicon nitride layer and thereby prevents theirrelease into the environment surrounding the engineered substrate, asillustrated in FIG. 2C.

Referring once again to FIG. 1, a bonding layer 120 (e.g., a siliconoxide layer) is deposited on a portion of the barrier layer 118, forexample, the top surface of the barrier layer, and subsequently usedduring the bonding of a substantially single crystal silicon layer 122.The bonding layer 120 can be approximately 1.5 μm in thickness in someembodiments.

The substantially single crystalline layer 122 is suitable for use as agrowth layer during an epitaxial growth process for the formation ofepitaxial material 130. In some embodiments, the epitaxial material 130includes a GaN layer 2 μm to 10 μm in thickness, which can be utilizedas one of a plurality of layers utilized in optoelectronic devices, RFdevices, power devices, and the like. In an embodiment, thesubstantially single crystalline layer 122 includes a substantiallysingle crystalline silicon layer that is attached to the silicon oxidelayer 118 using a layer transfer process.

FIG. 3 is a simplified schematic diagram illustrating an engineeredsubstrate structure according to an embodiment of the present invention.The engineered substrate 300 illustrated in FIG. 3 is suitable for avariety of electronic and optical applications. The engineered substrateincludes a core 110 that can have a coefficient of thermal expansion(CTE) that is substantially matched to the CTE of the epitaxial material130 that will be grown on the engineered substrate 300. Epitaxialmaterial 130 is illustrated as optional because it is not required as anelement of the engineered substrate structure, but will typically begrown on the engineered substrate structure.

For applications including the growth of gallium nitride (GaN)-basedmaterials (epitaxial layers including GaN-based layers), the core 110can be a polycrystalline ceramic material, for example, polycrystallinealuminum nitride (AlN). The thickness of the core can be on the order of100 to 1,500 μm, for example, 725 μm. The core 110 is encapsulated in afirst adhesion layer 112 that can be referred to as a shell or anencapsulating shell. In this implementation, the first adhesion layer112 completely encapsulates the core, but this is not required by thepresent invention, as discussed in additional detail with respect toFIG. 4.

In an embodiment, the first adhesion layer 112 comprises a tetraethylorthosilicate (TEOS) layer on the order of 1,000 Å in thickness. Inother embodiments, the thickness of the first adhesion layer varies, forexample, from 100 Å to 2,000 Å. Although TEOS is utilized for adhesionlayers in some embodiments, other materials that provide for adhesionbetween later deposited layers and underlying layers or materials can beutilized according to an embodiment of the present invention. Forexample, SiO₂, SiON, and the like adhere well to ceramic materials andprovide a suitable surface for subsequent deposition, for example, ofconductive materials. The first adhesion layer 112 completely surroundsthe core 110 in some embodiments to form a fully encapsulated core andcan be formed using an LPCVD process. The adhesion layer provides asurface on which subsequent layers adhere to form elements of theengineered substrate structure.

In addition to the use of LPCVD processes, furnace-based processes, andthe like to form the encapsulating adhesion layer, other semiconductorprocesses can be utilized according to embodiments of the presentinvention. As an example, a deposition process, for example, CVD, PECVD,or the like, that coats a portion of the core can be utilized, the corecan be flipped over, and the deposition process could be repeated tocoat additional portions of the core.

A conductive layer 314 is formed on at least a portion of the firstadhesion layer 112. In an embodiment, the conductive layer 314 includespolysilicon (i.e., polycrystalline silicon) that is formed by adeposition process on a lower portion (e.g., the lower half or backside)of the core/adhesion layer structure. In embodiments in which theconductive layer is polysilicon, the thickness of the polysilicon layercan be on the order of a few thousand angstroms, for example, 3,000 Å.In some embodiments, the polysilicon layer can be formed using an LPCVDprocess.

In an embodiment, the conductive layer 314 can be a polysilicon layerdoped to provide a highly conductive material, for example, theconductive layer 314 can be doped with boron to provide a p-typepolysilicon layer. In some embodiments, the doping with boron is at alevel ranging from about 1×10¹⁹ cm⁻³ to 1×10²⁰ cm⁻³ to provide for highconductivity. The presence of the conductive layer is useful duringelectrostatic chucking of the engineered substrate to semiconductorprocessing tools, for example tools with electrostatic chucks (ESC). Theconductive layer 314 enables rapid dechucking after processing. Thus,embodiments of the present invention provide substrate structures thatcan be processed in manners utilized with conventional silicon wafers.One of ordinary skill in the art would recognize many variations,modifications, and alternatives.

A second adhesion layer 316 (e.g., a second TEOS layer) is formedsurrounding the conductive layer 314 (e.g., a polysilicon layer). Thesecond adhesion layer 316 is on the order of 1,000 Å in thickness. Thesecond adhesion layer 316 can completely surround the conductive layer314 as well as the first adhesion layer 112 in some embodiments to forma fully encapsulated structure and can be formed using an LPCVD process.In other embodiments, second adhesion layer 316 only partially surroundsconductive layer 314, for example, terminating at the positionillustrated by plane 317, which may be aligned with the top surface ofconductive layer 314. In this example, the top surface of conductivelayer 314 will be in contact with a portion of barrier layer 118. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

A barrier layer 118 (e.g., a silicon nitride layer) is formedsurrounding the second adhesion layer 316. The barrier layer 118 is onthe order of 4,000 Å to 5,000 Å in thickness in some embodiments. Insome embodiments, the barrier layer 118 completely surrounds the secondadhesion layer 316 to form a fully encapsulated structure and can beformed using an LPCVD process.

In some embodiments, the use of a silicon nitride barrier layer preventsdiffusion and/or outgassing of elements present in the core 110, forexample, yttrium oxide (i.e., yttria), oxygen, metallic impurities,other trace elements and the like into the environment of thesemiconductor processing chambers in which the engineered substratecould be present, for example, during a high temperature (e.g., 1,000°C.) epitaxial growth process. Utilizing the encapsulating layersdescribed herein, ceramic materials, including polycrystalline AlN thatare designed for non-clean room environments can be utilized insemiconductor process flows and clean room environments.

FIG. 4 is a simplified schematic diagram illustrating an engineeredsubstrate structure according to another embodiment of the presentinvention. In the embodiment illustrated in FIG. 4, a first adhesionlayer 412 is formed on at least a portion of the core 110, but does notencapsulate the core 110. In this implementation, the first adhesionlayer 412 is formed on a lower surface of the core 110 (the backside ofthe core 110) in order to enhance the adhesion of a subsequently formedconductive layer 414 as described more fully below. Although adhesionlayer 412 is only illustrated on the lower surface of the core 110 inFIG. 4, it will be appreciated that deposition of adhesion layermaterial on other portions of the core will not adversely impact theperformance of the engineered substrates structure and such material canbe present in various embodiments. One of ordinary skill in the artwould recognize many variations, modifications, and alternatives.

The conductive layer 414 does not encapsulate the first adhesion layer412 and the core 110, but is substantially aligned with the firstadhesion layer 412. Although the conductive layer 414 is illustrated asextending along the bottom or backside and up a portion of the sides ofthe first adhesion layer 412, extension along the vertical side is notrequired by the present invention. Thus, embodiments can utilizedeposition on one side of the substrate structure, masking of one sideof the substrate structure, or the like. The conductive layer 414 can beformed on a portion of one side, for example, the bottom/backside, ofthe first adhesion layer 412. The conductive 414 layer provides forelectrical conduction on one side of the engineered substrate structure,which can be advantageous in RF and high power applications. Theconductive layer can include doped polysilicon as discussed in relationto the conductive layer 114 in FIG. 1.

A portion of the core 110, portions of the first adhesion layer 412, andthe conductive layer 414 are covered with a second adhesion layer 416 inorder to enhance the adhesion of the barrier layer 418 to the underlyingmaterials. The barrier layer 418 forms an encapsulating structure toprevent diffusion from underlying layers as discussed above.

In addition to semiconductor-based conductive layers, in otherembodiments, the conductive layer 414 is a metallic layer, for example,500 Å of titanium, or the like.

Referring once again to FIG. 4, depending on the implementation, one ormore layers may be removed. For example, layers 412 and 414 can beremoved, only leaving a single adhesion shell 416 and the barrier layer418. In another embodiment, only layer 414 can be removed. In thisembodiment, layer 412 may also balance the stress and the wafer bowinduced by layer 120, deposited on top of layer 418. The construction ofa substrate structure with insulating layers on the top side of Core 110(e.g., with only insulating layer between core 110 and layer 120) willprovide benefits for power/RF applications, where a highly insulatingsubstrate is desirable.

In another embodiment, the barrier layer 418 may directly encapsulatecore 110, followed by the conductive layer 414 and subsequent adhesionlayer 416. In this embodiment, layer 120 may be directly deposited ontothe adhesion layer 416 from the top side. In yet another embodiment, theadhesion layer 416 may be deposited on the core 110, followed by abarrier layer 418, and then followed by a conductive layer 414, andanother adhesion layer 412.

Although some embodiments have been discussed in terms of a layer, theterm layer should be understood such that a layer can include a numberof sub-layers that are built up to form the layer of interest. Thus, theterm layer is not intended to denote a single layer consisting of asingle material, but to encompass one or more materials layered in acomposite manner to form the desired structure. One of ordinary skill inthe art would recognize many variations, modifications, andalternatives.

FIG. 5 is a simplified flowchart illustrating a method of fabricating anengineered substrate according to an embodiment of the presentinvention. The method can be utilized to manufacture a substrate that isCTE matched to one or more of the epitaxial layers grown on thesubstrate. The method 500 includes forming a support structure byproviding a polycrystalline ceramic core (510), encapsulating thepolycrystalline ceramic core in a first adhesion layer forming a shell(512) (e.g., a tetraethyl orthosilicate (TEOS) shell), and encapsulatingthe first adhesion layer in a conductive shell (514) (e.g., apolysilicon shell). The first adhesion layer can be formed as a singlelayer of TEOS. The conductive shell can be formed as a single layer ofpolysilicon.

The method also includes encapsulating the conductive shell in a secondadhesion layer (516) (e.g., a second TEOS shell) and encapsulating thesecond adhesion layer in a barrier layer shell (518). The secondadhesion layer can be formed as a single layer of TEOS. The barrierlayer shell can be formed as a single layer of silicon nitride.

Once the support structure is formed by processes 510-518, the methodfurther includes joining a bonding layer (e.g., a silicon oxide layer)to the support structure (520) and joining a substantially singlecrystalline layer, for example, a substantially single crystallinesilicon layer, to the silicon oxide layer (522). Other substantiallysingle crystalline layers can be used according to embodiments of thepresent invention, including SiC, sapphire, GaN, AlN, SiGe, Ge, Diamond,Ga₂O₃, ZnO, and the like. The joining of the bonding layer can includedeposition of a bonding material followed by planarization processes asdescribed herein. In an embodiment as described below, joining thesubstantially single crystalline layer (e.g., a substantially singlecrystalline silicon layer) to the bonding layer utilizes a layertransfer process in which the layer is a single crystal silicon layerthat is transferred from a silicon wafer.

Referring to FIG. 1, the bonding layer 120 can be formed by a depositionof a thick (e.g., 4 μm thick) oxide layer followed by a chemicalmechanical polishing (CMP) process to thin the oxide to approximately1.5 μm in thickness. The thick initial oxide serves to fill voids andsurface features present on the support structure that may be presentafter fabrication of the polycrystalline core and continue to be presentas the encapsulating layers illustrated in FIG. 1 are formed. The CMPprocess provides a substantially planar surface free of voids,particles, or other features, which can then be used during a wafertransfer process to bond the substantially single crystalline layer 122(e.g., a substantially single crystalline silicon layer) to the bondinglayer 120. It will be appreciated that the bonding layer 120 does nothave to be characterized by an atomically flat surface, but shouldprovide a substantially planar surface that will support bonding of thesubstantially single crystalline layer (e.g., a substantially singlecrystalline silicon layer) with the desired reliability.

A layer transfer process can be used to join the substantially singlecrystalline silicon layer 122 to the bonding layer 120. In someembodiments, a silicon wafer (e.g., a silicon (111) wafer) is implantedto form a cleave plane. After wafer bonding, the silicon substrate canbe removed along with the portion of the single crystal silicon layerbelow the cleave plane, resulting in the exfoliated single crystalsilicon layer 122 illustrated in FIG. 1. The thickness of thesubstantially single crystal layer 122 can be varied to meet thespecifications of various applications. Moreover, the crystalorientation of the substantially single crystal layer 122 can be variedto meet the specifications of the application. Additionally, the dopinglevels and profile in the substantially single crystal layer 122 can bevaried to meet the specifications of the particular application.

The method illustrated in FIG. 5 may also include smoothing thesubstantially single crystal layer (524). In some embodiments, thethickness and the surface roughness of the substantially single crystallayer 122 can be modified for high quality epitaxial growth. Differentdevice applications may have slightly different specifications regardingthe thickness and surface smoothness of the substantially single crystallayer 122. The cleave process delaminates the substantially singlecrystal layer 122 from a bulk single crystal silicon wafer at a peak ofan implanted ion profile. After cleaving, the substantially singlecrystal layer 122 can be adjusted or modified in several aspects beforeit is utilized as a growth surface for epitaxial growth of othermaterials, such as gallium nitride.

First, the transferred substantially single crystal layer 122 maycontain a small amount of residual hydrogen concentration and may havesome crystal damage from the implant. Therefore, it may be beneficial toremove a thin portion of the transferred substantially single crystallayer 122 where the crystal lattice is damaged. In some embodiments, thedepth of the implant may be adjusted to be greater than the desiredfinal thickness of substantially single crystal layer 122. Theadditional thickness allows for the removal of the thin portion of thetransferred substantially single crystal layer that is damaged, leavingbehind the undamaged portion of the desired final thickness.

Second, it may be desirable to adjust the total thickness of thesubstantially single crystal layer 122. In general, it may be desirableto have the substantially single crystal layer 122 thick enough toprovide a high quality lattice template for the subsequent growth of oneor more epitaxial layers but thin enough to be highly compliant. Thesubstantially single crystal layer 122 may be said to be “compliant”when the substantially single crystal layer 122 is relatively thin suchthat its physical properties are less constrained and able to mimicthose of the materials surrounding it with less propensity to generatecrystalline defects. The compliance of the substantially single crystallayer 122 may be inversely related to the thickness of the substantiallysingle crystal layer 122. A higher compliance can result in lower defectdensities in the epitaxial layers grown on the template and enablethicker epitaxial layer growth. In some embodiments, the thickness ofthe substantially single crystal layer 122 may be increased by epitaxialgrowth of silicon on the exfoliated silicon layer.

Third, it may be beneficial to improve the smoothness of thesubstantially single crystal layer 122. The smoothness of the layer maybe related to the total hydrogen dose, the presence of any co-implantedspecies, and the annealing conditions used to form the hydrogen-basedcleave plane. The initial roughness resulting from the layer transfer(i.e., the cleave step) may be mitigated by thermal oxidation and oxidestrip, as discussed below.

In some embodiments, the removal of the damaged layer and adjusting thefinal thickness of the substantially single crystal layer 122 may beachieved through thermal oxidation of a top portion of the exfoliatedsilicon layer, followed by an oxide layer strip with hydrogen fluoride(HF) acid. For example, an exfoliated silicon layer having an initialthickness of 0.5 μm may be thermally oxidized to create a silicondioxide layer that is about 420 nm thick. After removal of the grownthermal oxide, the remaining silicon thickness in the transferred layermay be about 53 nm. During thermal oxidation, implanted hydrogen maymigrate toward the surface. Thus, the subsequent oxide layer strip mayremove some damage. Also, thermal oxidation is typically performed at atemperature of 1000° C. or higher. The elevated temperature can may alsorepair lattice damage.

The silicon oxide layer formed on the top portion of the substantiallysingle crystal layer during thermal oxidation can be stripped using HFacid etching. The etching selectivity between silicon oxide and silicon(SiO₂:Si) by HF acid may be adjusted by adjusting the temperature andconcentration of the HF solution and the stoichiometry and density ofthe silicon oxide. Etch selectivity refers to the etch rate of onematerial relative to another. The selectivity of the HF solution canrange from about 10:1 to about 100:1 for (SiO₂:Si). A high etchselectivity may reduce the surface roughness by a similar factor fromthe initial surface roughness. However, the surface roughness of theresultant substantially single crystal layer 122 may still be largerthan desired. For example, a bulk Si (111) surface may have aroot-mean-square (RMS) surface roughness of less than 0.1 nm asdetermined by a 2 μm×2 μm atomic force microscope (AFM) scan beforeadditional processing. In some embodiments, the desired surfaceroughness for epitaxial growth of gallium nitride materials on Si (111)may be, for example, less than 1 nm, less than 0.5 nm, or less than 0.2nm, on a 30 μm×30 μm AFM scan area.

If the surface roughness of the substantially single crystal layer 122after thermal oxidation and oxide layer strip exceeds the desiredsurface roughness, additional surface smoothing may be performed. Thereare several methods of smoothing a silicon surface. These methods mayinclude hydrogen annealing, laser trimming, plasma smoothing, and touchpolish (e.g., chemical mechanical polishing or CMP). These methods mayinvolve preferential attack of high aspect ratio surface peaks. Hence,high aspect ratio features on the surface may be removed more quicklythan low aspect ratio features, thus resulting in a smoother surface.

It should be appreciated that the specific steps illustrated in FIG. 5provide a particular method of fabricating an engineered substrateaccording to an embodiment of the present invention. Other sequences ofsteps may also be performed according to alternative embodiments. Forexample, alternative embodiments of the present invention may performthe steps outlined above in a different order. Moreover, the individualsteps illustrated in FIG. 5 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

FIG. 6 is a simplified schematic diagram illustrating anepitaxial/engineered substrate structure for RF and power applicationsaccording to an embodiment of the present invention. In some LEDapplications, the engineered substrate structure provides a growthsubstrate that enables the growth of high quality GaN layers and theengineered substrate structure is subsequently removed. However, for RFand power device applications, the engineered substrate structure formsportions of the finished device and as a result, the electrical,thermal, and other properties of the engineered substrate structure orelements of the engineered substrate structure are important to theparticular application.

Referring to FIG. 1, the single crystal silicon layer 122 is typicallyan exfoliated layer split from a silicon donor wafer using an implantand exfoliation technique. Typical implants are hydrogen and boron. Forpower and RF device applications, the electrical properties of thelayers and materials in the engineered substrate structure are ofimportance. For example, some device architectures utilize highlyinsulating silicon layers with resistance greater than 10³ Ohm-cm toreduce or eliminate leakage through the substrate and interface layers.Other applications utilized designs that include a conductive siliconlayer of a predetermined thickness (e.g., 1 μm) in order to connect thesource of the device to other elements. Thus, in these applications,control of the dimensions and properties of the single crystal siliconlayer is desirable. In design in which implant and exfoliationtechniques are used during layer transfer, residual implant atoms, forexample, hydrogen or boron, are present in the silicon layer, therebyaltering the electrical properties. Additionally, it can be difficult tocontrol the thickness, conductivity, and other properties of thinsilicon layers, using, for example, adjustments in the implant dose,which can impact conductivity as well as the full width at half max(FWHM) of the implant profile, surface roughness, and cleave planeposition accuracy, and implant depth, which can impact layer thickness.

According to embodiments of the present invention, silicon epitaxy on anengineered substrate structure is utilized to achieve desired propertiesfor the single crystal silicon layer as appropriate to particular devicedesigns.

Referring to FIG. 6, the epitaxial/engineered substrate structure 600includes an engineered substrate structure 610 and a silicon epitaxiallayer 620 formed thereon. The engineered substrate structure 610 can besimilar to the engineered substrate structures illustrated in FIGS. 1,3, and 4. Typically, the substantially single crystalline silicon layer122 is on the order of 0.5 μm after layer transfer. Surface conditioningprocesses can be utilized to reduce the thickness of the single crystalsilicon layer 122 to about 0.3 μm in some processes. In order toincrease the thickness of the single crystal silicon layer to about 1 μmfor use in making reliable Ohmic contacts, for example, an epitaxialprocess is used to grow epitaxial single crystal silicon layer 620 onthe substantially single crystalline silicon layer 122 formed by thelayer transfer process. A variety of epitaxial growth processes can beused to grow epitaxial single crystal silicon layer 620, including CVD,ALD, MBE, or the like. The thickness of the epitaxial single crystalsilicon layer 620 can range from about 0.1 μm to about 20 μm, forexample between 0.1 μm and 10 μm.

FIG. 7 is a simplified schematic diagram illustrating a III-V epitaxiallayer on an engineered substrate structure according to an embodiment ofthe present invention. The structure illustrated in FIG. 7 can bereferred to as a double epitaxial structure as described below. Asillustrated in FIG. 7, an engineered substrate structure 710 includingan epitaxial single crystal silicon layer 620 has a III-V epitaxiallayer 720 formed thereon. In an embodiment, the III-V epitaxial layercomprises gallium nitride (GaN).

The desired thickness of the III-V epitaxial layer 720 can varysubstantially, depending on the desired functionality. In someembodiments, the thicknesses of the III-V epitaxial layer 720 can varybetween 0.5 μm and 100 μm, for example, thicknesses greater than 5 μm.Resulting breakdown voltages of a device fabricated on the III-Vepitaxial layer 720 can vary depending on the thickness of the III-Vepitaxial layer 720. Some embodiments provide for breakdown voltages ofat least 100 V, 300 V, 600 V, 1.2 kV, 1.7 kV, 3.3 kV, 5.5 kV, 13 kV, or20 kV.

In order to provide for electrical conductivity between portions of theIII-V epitaxial layer 720, which can include multiple sub-layers, a setof vias 724 are formed passing, in this example, from a top surface ofthe III-V epitaxial layer 720, into the epitaxial single crystal siliconlayer 620. The vias 724 may be lined with an insulating layer (notshown) so that they are insulated from the III-V epitaxial layer 720. Asan example, these vias could be used to connect an electrode of a diodeor a transistor to the underlying silicon layer by providing an Ohmiccontact through the vias, thereby relaxing charge build up in thedevice.

If the III-V epitaxial layer were grown on the single crystal siliconlayer 122, it would be difficult to make such an Ohmic contact throughthe vias since terminating the via etch in the single crystal siliconlayer 122 would be difficult: for example, etching through 5 μm of GaNand terminating the etch in a 0.3 μm silicon layer reliably across anentire wafer. Utilizing embodiments of the present invention, it ispossible to provide single crystal silicon layers multiple microns inthickness, which is difficult using implant and exfoliation processessince achieving large implant depth requires high implant energy. Inturn, the thick silicon layers enable applications such as theillustrated vias that enable a wide variety of device designs.

In addition to increasing the thickness of the silicon “layer” byepitaxially growing the single crystal silicon layer 620 on the singlecrystal silicon layer 122, other adjustments can be made to the originalproperties of the single crystal silicon layer 122, includingmodifications of the conductivity, crystallinity, and the like. Forexample, if a silicon layer on the order of 10 μm is desired beforeadditional epitaxial growth of III-V layers or other materials, such athick layer can be grown according to embodiments of the presentinvention.

Because the implant process can impact the properties of the singlecrystal silicon layer 122, for example, residual boron/hydrogen atomscan influence the electrical properties of the silicon, embodiments ofthe present invention remove a portion of the single crystal siliconlayer 122 prior to epitaxial growth of single crystal silicon layer 620.For example, the single crystal silicon layer 122 can be thinned to forma layer 0.1 μm in thickness or less, removing most or all of theresidual boron/hydrogen atoms. Subsequent growth of single crystalsilicon layer 620 is then used to provide a single crystal material withelectrical and/or other properties substantially independent of thecorresponding properties of the layer formed using layer transferprocesses.

In addition to increasing the thickness of the single crystal siliconmaterial coupled to the engineered substrate structure, the electricalproperties including the conductivity of the epitaxial single crystalsilicon layer 620 can be different from that of the single crystalsilicon layer 122. Doping of the epitaxial single crystal silicon layer620 during growth can produce p-type silicon by doping with boron andn-type silicon by doping with phosphorus. Undoped silicon can be grownto provide high resistivity silicon used in devices that have insulatingregions. Insulating layers can be of use in RF devices, in particular.

The lattice constant of the epitaxial single crystal silicon layer 620can be adjusted during growth to vary from the lattice constant of thesingle crystal silicon layer 122 to produce strained epitaxial material.In addition to silicon, other elements can be grown epitaxially toprovide layers, including strained layers, that include silicongermanium, or the like. For instance, buffer layers can be grown on thesingle crystal silicon layer 122, on the epitaxial single crystalsilicon layer 620, or between layers to enhance subsequent epitaxialgrowth. These buffer layers could include strained III-V layers, silicongermanium strained layers, and the like. Additionally, the buffer layersand other epitaxial layers can be graded in mole fraction, dopants,polarity, or the like. One of ordinary skill in the art would recognizemany variations, modifications, and alternatives.

In some embodiments, strain present in the single crystal silicon layer122 or the epitaxial single crystal silicon layer 620 may be relaxedduring growth of subsequent epitaxial layers, including III-V epitaxiallayers.

FIG. 8 is a simplified flowchart illustrating a method of fabricating anengineered substrate according to another embodiment of the presentinvention. The method includes forming a support structure by providinga polycrystalline ceramic core (810), forming a first adhesion layercoupled to at least a portion of the polycrystalline ceramic core (812).The first adhesion layer can include a tetraethyl orthosilicate (TEOS)layer. The method also includes forming a conductive layer coupled tothe first adhesion layer (814). The conductive layer can be apolysilicon layer. The first adhesion layer can be formed as a singlelayer of TEOS. The conductive layer can be formed as a single layer ofpolysilicon.

The method also includes forming a second adhesion layer coupled to atleast a portion of the conductive layer (816), and forming a barriershell (818). The second adhesion layer can be formed as a single layerof TEOS. The barrier shell can be formed as a single layer of siliconnitride or a series of sub-layers forming the barrier shell.

Once the support structure is formed by processes 810-818, the methodfurther includes joining a bonding layer (e.g., a silicon oxide layer)to the support structure (820) and joining a substantially singlecrystalline silicon layer or a substantially single crystal layer to thesilicon oxide layer (822). The joining of the bonding layer can includedeposition of a bonding material followed by planarization processes asdescribed herein.

A layer transfer process can be used to join the substantially singlecrystalline silicon layer 122 to the bonding layer 120. In someembodiments, a silicon wafer (e.g., a silicon (111) wafer) is implantedto form a cleave plane. After wafer bonding, the silicon substrate canbe removed along with the portion of the single crystal silicon layerbelow the cleave plane, resulting in the exfoliated single crystalsilicon layer 122 illustrated in FIG. 1. The thickness of thesubstantially single crystalline silicon layer 122 can be varied to meetthe specifications of various applications. Moreover, the crystalorientation of the substantially single crystal layer 122 can be variedto meet the specifications of the application. Additionally, the dopinglevels and profile in the substantially single crystal layer 122 can bevaried to meet the specifications of the particular application. In someembodiments, the substantially single crystalline silicon layer 122 canbe smoothed, as described above.

The method illustrated in FIG. 8 may also include forming an epitaxialsilicon layer by epitaxial growth on the substantially singlecrystalline silicon layer (824), and forming an epitaxial III-V layer byepitaxial growth on the epitaxial silicon layer (826). In someembodiments, the epitaxial III-V layer may comprise gallium nitride(GaN).

It should be appreciated that the specific steps illustrated in FIG. 8provide a particular method of fabricating an engineered substrateaccording to another embodiment of the present invention. Othersequences of steps may also be performed according to alternativeembodiments. For example, alternative embodiments of the presentinvention may perform the steps outlined above in a different order.Moreover, the individual steps illustrated in FIG. 8 may includemultiple sub-steps that may be performed in various sequences asappropriate to the individual step. Furthermore, additional steps may beadded or removed depending on the particular applications. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A method of manufacturing a substrate, the methodcomprising: forming a support structure by: providing a polycrystallineceramic core; encapsulating the polycrystalline ceramic core in a firstadhesion shell; encapsulating the first adhesion shell in a conductiveshell; encapsulating the conductive shell in a second adhesion shell;and encapsulating the second adhesion shell in a barrier shell; joininga bonding layer to the support structure; joining a substantially singlecrystalline silicon layer to the bonding layer; forming an epitaxialsilicon layer by epitaxial growth on the substantially singlecrystalline silicon layer; and forming one or more epitaxial III-Vlayers by epitaxial growth on the epitaxial silicon layer.
 2. The methodof claim 1 further comprising forming a plurality of vias passing fromthe one or more epitaxial III-V layers to the epitaxial silicon layer.3. The method of claim 1 wherein the polycrystalline ceramic corecomprises aluminum nitride.
 4. The method of claim 1 wherein the one ormore epitaxial III-V layers comprise an epitaxial gallium nitride layer.5. The method of claim 4 wherein the one or more epitaxial III-V layersfurther comprise an epitaxial aluminum nitride layer, or an epitaxialaluminum gallium nitride layer, or a combination thereof.
 6. The methodof claim 1 wherein: the first adhesion shell comprises a firsttetraethyl orthosilicate (TEOS) shell; the conductive shell comprises apolysilicon shell; the second adhesion shell comprises a second TEOSshell; the barrier shell comprises a silicon nitride shell; and thebonding layer comprises silicon oxide.
 7. The method of claim 6 wherein:the first TEOS shell comprises a single layer of TEOS; the polysiliconshell comprises a single layer of polysilicon; the second TEOS shellcomprises a single layer of TEOS; and the silicon nitride shellcomprises a single layer of silicon nitride.
 8. The method of claim 1wherein a thickness of the one or more epitaxial III-V layers is about 5μm or greater.
 9. The method of claim 1 wherein joining thesubstantially single crystalline silicon layer is performed byexfoliation.
 10. The method of claim 9 wherein the substantially singlecrystalline silicon layer has a thickness of about 0.5 μm.